Feedback control systems typically use sensors to measure states of the target system to be controlled by the control system. For example, optical sensors and Hall effect devices produce rotor position signals in feedback control systems for brushless direct current motors. However, such sensors add cost and complexity to a system and may also require maintenance from time to time to assure continued proper operation. Such sensors can also be a common point of failure in systems under feedback control.
As a result of the disadvantages of many sensor devices, sensorless feedback control systems, which are not based on direct sensing of target system states, are attractive for some applications. For example, the back electromotive force (EMF) generated by stator windings of a brushless DC motor as its magnetized rotor rotates can be detected and used to determine rotor position. The transitions in the resulting back-EMF signal indicate times at which the rotor is in known positions.
Existing feedback control systems drive the phase error, i.e. the difference between a command signal and the target system's response to the command signal, toward zero. Such a feedback control system is point optimized. However, rather than simply nulling the phase error and converging to a single operating point, it would be advantageous for a feedback control system to be able to track an error signal function, which is the difference between the actual state and desired state of the system, in order to deliberately run the motor with a non-zero phase error. The present invention provides a feedback control technique which provides this capability.